The development of the magnetohydrodynamic (MHD) generator has been considered important because of several advantages. These include a low capital cost, greater efficiency of heat conversion to electrical energy and rapid start-up. The development of the MHD generator has been hampered by several problems, especially in the case of coal combustion. Among these problems are the effect of ash on the process equipment, the removal of the ash and separation of the ash or coal slag from the seed. Low combustion temperatures which require the necessity of preheating air to the burner and the low overall MHD efficiency due to the high energy content of the effluent gases from the MHD generator pose additional problems.
In general terms, MHD generators produce electrical power by movement of electrically conductive fluid relative to a magnetic field. The fluid employed is usually an electrically conductive gas from a high temperature, high pressure source. From the source, the fluid flows through the generator and, by virture of its movement relative to the magnetic field, induces an electromotive force between opposed electrodes in the generator. The gas may exhaust to a sink which may simply be the atmosphere; or, in a more sophisticated system, the gas may exhaust to a recovery system including pumping mechanism for returning the gas to the source.
Several different gases may be used; the gas may be products of combustion, or may comprise inert gases such as helium or argon. In open systems, such as those in which the gases are not recycled after passing through the power plant, products of combustion are normally used. In closed systems, in which the gases are recycled, it is feasible to use relatively expensive gases, such as helium and argon. To promote electrical conductivity, the gases are heated to a high temperature; conductivity is also increased by the addition to the gases of a substance that ionizes readily at the operating temperatures of the generator. Regardless of the gas used, the gas includes a mixure of electrons, positive ions and neutral atoms which, for convenience, is usually termed "plasma".
The temperature of the plasma is highly significant, not only to the overall efficiency of the system but also to the design of the MHD generator. With a higher temperature available at the inlet of the generator, a larger isentropic drop can be developed as the plasma expands through the generator, resulting in an improved plant efficiency. Because the electrical conductivity of the plasma increases as the temperature increases, it is possible to generate a given amount of power in a relatively smaller generator and employ a smaller magnetic field than would otherwise be possible with employment of increased temperatures. The increased efficiency of the MHD system, considerably above that of conventional stream turbine plants, coupled with the absence of hot moving parts in the generator suggest that in time MHD power plants will replace or substantially supplant power generating systems of conventional design.
Some of the problems endemic to MHD systems, even after the substantial amount of development work over the past several years includes the loss of high energy gas from the MHD generator as well as the necessity to preheat air in order to obtain the requisite high temperatures at the generator inlet and to dry and preheat the fuel, particularly where coal is employed.
Representative literature relating to MHD generating systems includes U.S. Pat. No. 3,414,744 issued Dec. 3, 1968 to Petrick for Magnetohydrodynamic Generator which discloses the use of an MHD generator using NaK coolant from a nuclear reactor.
U.S. Pat. No. 3,531,665 issued Sept. 29, 1970 to Rosa for Coal Preheating System for Magnetohydrodynamic Devices which discloses mechanism for preheating pulverized coal with MHD off gas.
U.S. Pat. No. 3,720,850 issued Mar. 13, 1973 to Way for Magnetohydrodynamic Power System With Semi-Closed Cycle shows the recycling of MHD off gases to the inlet side of the MHD generator.
U.S. Pat. No. 3,873,845 issued Mar. 25, 1975 to Osthaus for Method Of Producing Electric Energy Including Coal Gasification discloses a process and system for gasifying coal dust with air heated to 1500.degree. C., the combustion gas therefrom being cooled to 150.degree. C. thereby producing high pressure steam for producing electricity.
U.S. Pat. No. 3,895,243 issued July 15, 1975 to Amend et al. for Method And Means Of Generating Power From Fossil Fuels With A Combined Plasma And Liquid-Metal MHD Cycle discloses a process for utilizing the waste heat from a fossil fuel MHD generator to heat a liquid-metal MHD generator. Air is preheated by heat exchange with the walls of the combustion chamber for the MHD generator.
U.S. Pat. No. 4,064,222 issued Dec. 20, 1977 to Bretz for Nitrogen Fixation And Molecular Magneto Hydrodynamic Generator Using A Coal Gasification Gas Stream discloses a coal gasifier using coal and oxygen to produce off gas which is burned with air and fed to a MHD generator followed by adiabatic expansion to fix the nitrogen oxides.
U.S. Pat. No. 4,107,557 issued Aug. 15, 1978 to Shepard for Sulfur-Fueled Magnetohydrodynamic Power Generation discloses a closed cycle MHD generator using sulfur and oxgen to produce a flame temperature of greater than 8000.degree. F. to the MHD generator.